One type of field effect transistor (FET) used for power switching is an enhancement mode type FET. An enhancement mode FET is normally non-conducting. However, when a gate voltage above a threshold value is applied, the enhancement mode FET becomes conducting. Additionally, enhancement mode FETs are available in two gate polarities: N-channel and P-channel.
Power switching circuits designed for general purpose use are usually constructed with N-channel FETs because, for any given die size, the N-channel FET has a lower ON resistance than a correspondingly sized P-channel FET would have.
For clarity, several terms used herein are defined. The term “radiation hardened” in the context of the present invention is understood to refer to components and/or circuits, which have been explicitly designed and/or tested to operate at a specified operating point under defined radiation levels and exposure duration. In contrast, “non-radiation hardened” electronic components are standard, commercially available components that have not been tested or rated for operation in radiation environments.
Moreover, radiation environment within the context of the present invention refers to an operating environment of a circuit in which the circuit will be exposed to one or more forms of electromagnetic radiation, for example, ionizing radiation, that generally cause deterioration to electronic components.
The conventional switching devices in radiation environments use specially designed radiation hardened N or P-channel FETs. The principal benefit of these radiation hardened N or P-channel FETs is that the gate threshold voltage does not change substantially after being exposed to radiation. However, because these components are in limited demand, the supply is correspondingly lower than for similar commercial-grade components. The limited supply for radiation hardened components leads to increased cost and may increase production lead times. These factors result in higher prices and longer delivery times for the radiation tolerant circuits that incorporate these types of components.
If a conventional non-radiation hardened N-channel FET is used in switching applications where radiation is present, the device tends to fail at relatively low radiation levels because the gate threshold voltage of the N-channel FET shifts more negatively with accumulated radiation dose, and ultimately falls through zero to a negative voltage. Below a zero voltage threshold, the N-channel FET conducts current with little or no gate voltage applied. Therefore, the component is difficult to control after the initial application of power.
The gate threshold voltage of a conventional, non-radiation hardened P-channel FET also shifts more negative as it is exposed to accumulated radiation dose. However, the initial threshold voltage is negative. Therefore, the gate threshold voltage never goes through a region where the FET is uncontrollable, it only shifts from a negative value to a more negative value. Consequently, conventional P-channel FETs can be more immune to total dose effects than conventional N-channel FETs if the proper gate drive signal is provided.
Therefore, it is desirable to provide a power switching circuit using an N-channel FET that minimizes the deleterious effects described above in a radiation environment.